If the model is linear, then you can get the input load Fin required to get a 500 lb reaction force output after you have solved the model one time using a 1000 lb input load and obtaining the reaction force output RFout by a simple ratio:

Fin = 1000*RFout/500

This assumes that RFout = 0 when Fin = 0. If there is an offset, such as there is also a gravity load that causes RFout to be non-zero when Fin = 0, then you just solve with Fin = 0 to get that offset value and subtract it from the equation.

If the model is nonlinear, then you use the same formula iteratively to get closer and closer to the value of Fin that gets you as close to 500 lb as you want.

You could also make the input load a parameter, an the reaction force output a parameter and use the Design Exploration module Direct Optimization and let Ansys automatically search for the value of Fin that delivers a Target value of RFout within a tolerance range.